I do not think they should be marked. It would make the game less interesting, as it is now, you never know when you're going to get beaten. A few days ago I came across a couple mammoths, I thought they might go down as easy as everything else I had encountered out on the tundra. Not so, they charged me, then the GIants, whom I did not see, joined in as well. Point being, nice to have surprises now and again.